During a 100-metre sprint, the immediate energy for cross-bridge cycling in type IIx fibres comes primarily from which source?

• A Oxidative phosphorylation of fatty acids
• B Transfer of a phosphate group from creatine phosphate to ADP ✓
• C Anaerobic glycolysis beginning from stored glycogen
• D Direct use of preformed ATP released from myoglobin-bound stores

Explanation

The phosphagen system supplies ATP fastest but has the smallest capacity. Creatine phosphate donates its phosphate to ADP via creatine kinase, buffering ATP within seconds of the start of maximal effort. Anaerobic glycolysis becomes the dominant supplier only after several seconds, and oxidative metabolism contributes later still. There is no meaningful myoglobin-bound ATP store, making option D physiologically false rather than merely less important.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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